The present invention relates to a convolver arrangement with acoustic waves running in a substrate surface.
Convolvers for acoustic waves are known, for example, from Proc. IEEE, Ultrasonic Symposium (1974), pages 224-227 and (1981), pages 181-185.
Acoustic waves employed in conjunction with such electrical arrangements involve acoustic waves which run in a substrate close to the surface or in the surface. Such acoustic waves are known as Rayleigh Waves, Bluestein Waves, Love Waves, SSBW Waves, SABW Waves and the like, and which shall be referred to in general below as surface waves (although only the two first types of waves are surface waves in the strictest sense).
A surface wave convolver is an electrical device for extremely high frequencies, particularly beginning with the MHz range. Such a convolver is employed for processing, for example, binary orthogonal keying (BOK) signals.
A convolver is a combination of a plurality of structures arranged on the one surface of a substrate, for example, lithium niobate. These include an interdigital structure as an input transducer for the input signal to be processed. Also included is an interdigital transducer as an input transducer for a reference signal. These two input transducers are arranged opposite one another in the direction of or along the axis of the track of the acoustic waves. The integration electrode is situated between them, this integration electrode usually being a strip arranged on the substrate surface. The width of the integration electrode measured at right angles to the axis of the track of the wave propagation is usually considerably smaller than the width or finger length of the input digital transducer measured parallel thereto. For matching or adapting these extremely different input or output apertures of the input transducer and the integration electrode residing opposite one another, a beam compression structure is inserted between these structures for matching the apertures. The electrical output of such a convolver is a terminal connected to the integration electrode. From an input signal and a reference signal, such a convolver supplies a convolution signal of these two input signals.
Disturbing effects which are based on undesired, additional functions of individual or of a plurality of structures of the arrangement occur in surface wave arrangements as well as in convolvers. For example, reflections of the acoustic waves and the transducer fingers are known as a disturbing effect. A technique which is effective against these reflections is the formation of those interdigital transducers which can effect the disturbing appearance of reflections as split-finger structures. Specifically, in a convolver a self-convolution signal can occur as a noise signal, i.e., a convolution signal is formed of the acoustic wave of the signal proceeding as desired in the one direction of the convolver and of the wave of this input signal reflected at the input transducer for the reference signal which undesirably runs in the opposite direction. In order to eliminate this type of disturbance, two convolver structures as set forth above have been arranged together on one and the same surface of the substrate body, their individual structures being electrically connected to one another such that an elimination of the self-convolution signal is achieved. This electrical circuit diagram is essentially comprised such that the two input transducers for the input signal or for the reference signal are connected parallel to one another. The two integration electrodes are likewise connected in parallel. These two parallel-interconnected convolver structures interact such that reflections at the input transducers are prevented in the final result. This corresponds to a suppression of the regeneration effect and can be achieved by a corresponding geometrical arrangement of inter-related input transducers for the reference signal and integration electrode which is effective for the acoustic wave, namely a geometrical arrangement differing by Lambda/2 in the final result. For example, the respective spacings between the end of the integration electrode and the beginning of the input transducer for the reference signal can differ in size by this value Lambda/2 (or uneven multiples of Lambda/2). An equivalent technique is to provide such a spacing difference at the input side for the input signal. A likewise equivalent technique is to fashion the two interdigital transducers for the input signal or for the reference signal which form a pair of input transducers in terms of their interdigital structure such that they respectively emit such an acoustic wave, i.e., together emit waves between which a 180.degree. phase shift is present. What is achieved by means of such techniques is that the waves incident on a transducer pair always generate signals therein whose sum is zero. A regeneration, i.e., a reemission of waves, is thus prevented, these waves potentially appearing as a consequence of a voltage induced at the terminating impedance which terminates the interconnection of a transducer pair.
Corresponding matching networks are required for the feed of the input signal and of the reference signal. It is obviously advantagous if identical matching networks, namely the simplest possible matching networks, could be used for the input signal and for the reference signal. The simplest matching network is an inductance. This is not a problem both for a simple convolver as well as a convolver as set forth above comprising two convolver structures for compensation of the self-convolution signal. Either only one individual input transducer is provided at each end, or two input transducers connected in parallel which form a transducer pair are provided. Let it be pointed out that such a parallel connection can also be structurally constructed, i.e., instead of two individual (input) interdigital transducers connected in parallel, a single interdigital transducer (essentially twice as broad) is employable. This transducer is the input transducer for the two convolver structures (of the convolver with compensation of the self-convolution). Instead of such an interdigital transducer dimensioned twice as broad, it can also be provided that an interposed multi-strip coupler is inserted, this causing a coupling between the input transducer and the two convolver structures which has a compensating effect in view of the self-convolution signal.
Corresponding matching networks which are not to be included among the surface wave convolvers in the narrowest sense are required for the feed of the input signal and of the reference signals. However, it is necessary that the corresponding transducer of the convolvers is matched to the matching network, or that the respective matching network is matched to the corresponding convolver input transducer. This leads to a corresponding multiplicity.